Merge branch 'develop'

fix: add -Wmaybe-uninitialized pragma to union storage constructors

doc/00_optional.qbk

@@ -1,8 +1,8 @@
 [library Boost.Optional
-    [quickbook 1.4]
+    [quickbook 1.7]
     [authors [Cacciola Carballal, Fernando Luis]]
     [copyright 2003-2007 Fernando Luis Cacciola Carballal]
-    [copyright 2014-2026 Andrzej Krzemieński]
+    [copyright 2014-2026 Andrzej Krzemieński]
     [category miscellaneous]
     [id optional]
     [dirname optional]

doc/11_development.qbk

@@ -50,6 +50,18 @@ In the general case, the internal representation is something equivalent to:
     alignas(T) char _value [sizeof(T)];
   };
 
+or:
+
+  template <typename T>
+  class Optional
+  {
+    bool _has_value = false;
+    union {
+      T         _value;
+      DummyType _non_value;
+    };
+  };
+
 Next, because we need to pass around these "optional" `int`s as normal `int`s,
 like returning them from functions, when copying, we need to copy `_has_value`,
 which indicates whether we have the value or not, and, if we do have value, and

doc/16_optional_references.qbk

@@ -27,7 +27,7 @@ rather than the reference itself.
 On compilers that do not conform to Standard C++ rules of reference binding,
 some operations on optional references are disabled in order to prevent subtle
 bugs. For more details see
-[link boost_optional.dependencies_and_portability.optional_reference_binding Dependencies and Portability section].
+[link optional_reference_binding Dependencies and Portability section].
 ]
 
 [heading Rvalue references]
@@ -39,6 +39,8 @@ Rvalue references and lvalue references to const have the ability in C++ to exte
 
 [endsect]
 
+
+[#optional_ref_rebinding_semantics]
 [section Rebinding semantics for assignment of optional references]
 
 If you assign to an ['uninitialized ] `optional<T&>` the effect is to bind (for

doc/17_in_place_factories.qbk

@@ -1,5 +1,5 @@
 
-[section In-Place Factories]
+[section:in_place_factories In-Place Factories][#boost_optional_factories]
 
 One of the typical problems with wrappers and containers is that their
 interfaces usually provide an operation to initialize or assign the
@@ -138,4 +138,4 @@ The factories are implemented in the headers: __IN_PLACE_FACTORY_HPP__ and __TYP
 
 [caution The support for in-place factories is deprecated. Use constructor taking `in_place_init` tag and function `.emplace()` instead.]
 
-[endsect]
+[endsect:in_place_factories]

doc/1A_type_requirements.qbk

@@ -8,7 +8,7 @@ The very minimum requirement of `optional<T>` is that `T` is a complete type and
     assert(!o);        //
     o.value();         // always throws
 
-But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional.design.in_place_factories In-Place Factories]:
+But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional_factories In-Place Factories]:
 
     optional<T> o;
     o.emplace("T", "ctor", "params");

doc/21_ref_none.qbk

@@ -2,7 +2,7 @@
     Boost.Optional
 
     Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
-    Copyright (c) 2015 Andrzej Krzemienski
+    Copyright (c) 2015 Andrzej Krzemieński
 
     Distributed under the Boost Software License, Version 1.0.
     (See accompanying file LICENSE_1_0.txt or copy at
@@ -15,14 +15,25 @@
 ```
 namespace boost {
 
-class none_t {/* see below */};
+class none_t
+{
+  friend constexpr bool operator==(none_t, none_t) = default;
+  /* see below */
+};
 
 inline constexpr none_t none (/* see below */);
 
 } // namespace boost
 ```
 
-Class `none_t` is meant to serve as a tag for selecting appropriate overloads of from `optional`'s interface. It is an empty, trivially copyable class with disabled default constructor.
+Class `none_t` is meant to serve as a tag for selecting appropriate overloads of from `optional`'s interface.
+It is an empty class.
+
+It is [@https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable.html trivially copyable].
+
+It is ['not] [@https://en.cppreference.com/w/cpp/named_req/DefaultConstructible default-constructible].
+
+It models concept [@https://en.cppreference.com/w/cpp/concepts/equality_comparable `std::equality_comparable`].
 
 Constant `none` is used to indicate an optional object that does not contain a value in initialization, assignment and relational operations of `optional`.
 

doc/27_ref_optional_synopsis.qbk

@@ -197,6 +197,9 @@ They are empty, trivially copyable classes with disabled default constructor.
         template<class F> constexpr auto flat_map( F f ) & -> ``['see below]``; ``[link reference_optional_flat_map __GO_TO__]``
         template<class F> constexpr auto flat_map( F f ) && -> ``['see below]``; ``[link reference_optional_flat_map_move __GO_TO__]``
 
+        constexpr operator optional<T&>() & noexcept; ``[link reference_optional_conversion_to_ref __GO_TO__]``
+        constexpr operator optional<T const&>() const& noexcept; ``[link reference_optional_conversion_to_ref __GO_TO__]``
+
         T const* get_ptr() const ; ``[link reference_optional_get_ptr __GO_TO__]``
         T*       get_ptr() ; ``[link reference_optional_get_ptr __GO_TO__]``
 

doc/28_ref_optional_semantics.qbk

@@ -2,6 +2,7 @@
     Boost.Optional
 
     Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
+    Copyright (C) 2014 - 2026 Andrzej Krzemieński.
 
     Distributed under the Boost Software License, Version 1.0.
     (See accompanying file LICENSE_1_0.txt or copy at
@@ -25,31 +26,31 @@ __SPACE__
 
 [: `constexpr optional<T>::optional() noexcept;`]
 
-* [*Effect:] Default-Constructs an `optional`.
-* [*Postconditions:] `*this` is [_uninitialized].
-* [*Notes:] T's default constructor [_is not] called.
+* [*Postconditions:] `*this` does ['not] contain a value (is "uninitialized").
+* [*Remarks:] No contained value is initialized. For every object type `T` these constructors are core constant expressions.
 * [*Example:]
 ``
-optional<T> def ;
-assert ( !def ) ;
+optional<T> oN ;
+assert ( !oN ) ;
 ``
 
 __SPACE__
 
-[#reference_optional_constructor_none_t]
 
+[#reference_optional_constructor_none_t]
 [: `constexpr optional<T>::optional( none_t ) noexcept;`]
 
-* [*Effect:] Constructs an `optional` uninitialized.
-* [*Postconditions:] `*this` is [_uninitialized].
-* [*Notes:] `T`'s default constructor [_is not] called. The expression
+* [*Postconditions:] `*this` does ['not] contain a value (is "uninitialized").
+* [*Remarks:] No contained value is initialized. For every object type `T` these constructors are core constant expressions. The expression
 `boost::none` denotes an instance of `boost::none_t` that can be used as
 the parameter.
 * [*Example:]
 ``
 #include <boost/none.hpp>
+
 optional<T> n(none) ;
 assert ( !n ) ;
+assert ( n == none ) ;
 ``
 
 __SPACE__
@@ -281,7 +282,7 @@ factory.
 * [*Postconditions: ] `*this` is [_initialized] and its value is ['directly given]
 from the factory `f` (i.e., the value [_is not copied]).
 * [*Throws:] Whatever the `T` constructor called by the factory throws.
-* [*Notes:] See [link boost_optional.design.in_place_factories In-Place Factories]
+* [*Notes:] See [link boost_optional_factories In-Place Factories]
 * [*Exception Safety:] Exceptions can only be thrown during the call to
 the `T` constructor used by the factory; in that case, this constructor has
 no effect.
@@ -524,7 +525,7 @@ factory.
 * [*Postconditions: ] `*this` is [_initialized] and its value is ['directly given]
 from the factory `f` (i.e., the value [_is not copied]).
 * [*Throws:] Whatever the `T` constructor called by the factory throws.
-* [*Notes:] See [link boost_optional.design.in_place_factories In-Place Factories]
+* [*Notes:] See [link boost_optional_factories In-Place Factories]
 * [*Exception Safety:] Exceptions can only be thrown during the call to
 the `T` constructor used by the factory; in that case, the `optional` object
 will be reset to be ['uninitialized].
@@ -752,6 +753,26 @@ __SPACE__
 
 __SPACE__
 
+
+[#reference_optional_conversion_to_ref]
+
+[: `constexpr optional<T>::operator optional<T&>() & noexcept ;`]
+
+* [*Returns:] If `*this` contains a value `optional<T&>(**this)`, otherwise `optional<T&>()`.
+
+[: `constexpr optional<T>::operator optional<T const&>() const& noexcept ;`]
+
+* [*Returns:] If `*this` contains a value `optional<T const&>(**this)`, otherwise `optional<T&>()`.
+
+* [*Example:]
+``
+const optional<int> oi = 1;
+optional<const int&> ri = oi;
+``
+
+__SPACE__
+
+
 [#reference_optional_get_value_or_value]
 
 [: `T const& optional<T>::get_value_or( T const& default) const ;`]
@@ -956,7 +977,7 @@ __SPACE__
 
 * [*Postconditions:] `bool(*this) == bool(rhs)`.
 
-* [*Notes:] This behaviour is called ['rebinding semantics]. See [link boost_optional.design.optional_references.rebinding_semantics_for_assignment_of_optional_references here] for details.
+* [*Notes:] This behaviour is called ['rebinding semantics]. See [link optional_ref_rebinding_semantics here] for details.
 
 * [*Example:]
 ``

doc/29_ref_optional_convenience.qbk

@@ -11,7 +11,7 @@
 
 [section Header <boost/optional.hpp>]
 
-This is an alias for header [link boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional `<boost/optional/optional.hpp>`].
+This is an alias for header [link ref_header_optional_optional_hpp `<boost/optional/optional.hpp>`].
 
 
 [endsect]

doc/90_dependencies.qbk

@@ -13,13 +13,55 @@
 
 
 [section Minimum System Requirements]
-This library requires C++11 as minimum. However some features are disabled.
+This library requires C++11 as minimum. However, in C++11 some features are disabled.
 
-For C++14 and higher this library provides a `constexpr` interface for non-mutable
-member functions and some mutable member functions.
+[section:constexpr Support for `constexpr`]
+
+[section C++11]
+For compilers fully supporting C++11 (including unconstrained unions and ref-qualifiers),
+for trivially-destructible `T`s, `optional<T>` is a ['literal type] and its constructors
+with `this->has_value() == false` as postcondition:
+
+* `optional()`,
+* `optional(none_t)`,
+
+are ['core constant expressions]. Even for other `T`s, these constructors are guaranteed to
+perform ['constant initialization]: they are never subject to "initialization order fiasco".
+
+Other constructors with `this->has_value() == true`
+as postcondition are core constant expressions if the expression required to initialize
+the contained value is a core constant expression. This includes constructors:
+
+* `template <typename... Args> optional(in_place_init, Args&&...)`,
+* `template <typename U> optional(U&&)`,
+* `optional(const T&)`,
+* `optional(T&&)`.
+
+Other constructors, including the copy and move constructos, are ['not] core constant expressions.
+
+Member functions `.has_value`, `operator bool` and (non)equality comparisons with `none` are core-constant expressions for trivially-destructible `T`s.
+
+Also all `const`-qualified non-static member functions and comparison operators are core constant expressions, if the corresponding operations on `T`
+are core constant expressions.
+[endsect]
+
+[section C++14]
+For C++14 and higher this library, for trivially-destructible `T`s provides the `constexpr` interface for all mutable and non-mutable
+member functions, as long as:
+
+* the corresponding operations on `T` are core constant expressions and
+* the member never attempts to change the `optional`'s state from not containing a value to containing a value.
 
 [note For types that overload unary `operator&` (address) some member functions in `optional`, like `operator->`,
 cannot be implemented as `constexpr` on some compilers.]
+[endsect]
+
+[section C++17]
+In C++17 all non-deprecated constructors are core constant expressions as long as
+`T` is a literal type and its constructor used is a core constant expression.
+[endsect]
+
+[endsect:constexpr]
 
 [endsect]
 
@@ -30,7 +72,6 @@ The implementation uses the following other Boost modules:
 # assert
 # config
 # core
-# static_assert
 # throw_exception
 # type_traits
 

doc/91_comparison_with_std.qbk

@@ -11,8 +11,7 @@
 
 [section:std_comp Comparison with `std::optional`]
 
-[table
-  []
+[table Comparison with `std::optional`
   [ [[*`boost::optional`]] [[*`std::optional`]] [] ]
   [ [`optional<int> o = none;`] [`optional<int> o = nullopt;`] [Different name for no-value tag.] ]
   [ [`optional<X> o {in_place_init, a, b};`] [`optional<int> o {in_place, a, b};`] [Different name for in-place initialization tag.] ]
@@ -42,7 +41,23 @@
   [ [`make_optional(cond, v);`] [] [No `make_optional` with condition in `std`.] ]
   [ [] [`make_optional<T>(a, b);`] [No `make_optional` with specified `T` in `boost`.] ]
   [ [`std::cout << optional<int>{};`] [] [No printing to IOStreams in `std`.]]
+
+  [ [`std::vector<optional<int>> rng;`
+
+  `std::ranges::find(rng, boost::none)`] [`std::vector<optional<int>> rng;`
+
+  `std::ranges::find(rng, boost::optional<int>{})`] [optional<T> is never [@https://en.cppreference.com/w/cpp/concepts/equality_comparable `std::equality_comparable_with`] `nullopt_t` in `std`. ] ]
+
+  [ [```
+  void test(vector<optional<T>> rng, T val) {
+    std::ranges::find(rng, val);
+  }
+  ```] [```
+  void test(vector<optional<T>> rng, T val) {
+    std::ranges::find(rng, make_optional(val));
+  }
+  ```] [For `std::optional`, code without `make_optional()` compiles but is ['undefined behavior] when `T` is itself an `optional`. ] ]
+
 ]
 
-
 [endsect][/ std_comp]

doc/92_relnotes.qbk

@@ -11,9 +11,56 @@
 
 [section:relnotes Release Notes]
 
+[heading Boost Release 1.91]
 
+* For compilers with full C++11 support (including "unrestricted  unions" and ref-qualifiers)
+  changed the implementation from aligned storage to union storage. This enables the gradual
+  `constexpr` support:
 
+    * In C++11, some constructors and `const`-qualified accessors become usable
+      in compile-time contexts (are ['core constant expressions]) for types
+      satisfying certain constraints.
+    * In C++14 even some mutating operations become core constant expressions (those
+      that do not require changing the state of `optional` from not having a value to
+      having a value), for co-operating types.
+    * In C++17 all constructors (including copy and move) become core constant expressions
+      for co-operating types.
 
+  This addresses issues [@https://github.com/boostorg/optional/issues/132 #132] and [@https://github.com/boostorg/optional/issues/143 #143].
+
+* *Breaking change.* In the said implementation, abandoned the mechanism for customizing
+  `swap`. Hardly anyone knows about this mechanism and it was never documented.
+
+* In the said implementation, applied a couple of small changes to get closer to the interface of `std::optional`:
+  * Enabled syntax `o = {}` for putting optional objects to no-value state.
+  * Enabled syntax `o.value_or({})`, which uses a default-constructed `T`.
+  * Construct `o = u`, where `o` is of type `optional<T>` and `u` is of type `U` convertible to `T`,
+    does not create a temporary `T`.
+
+* In the said implementation, added a conversion from `optional<T>&` to `optional<T&>`. This addresses [@https://github.com/boostorg/optional/issues/142 issue #142].
+
+* `none_t` is now `std::equality_comparable`, which means that `none_t` and `optional<T>`
+  model concept `std::equality_comparable_with` (for `std::equality_comparable` `T`s),
+  which means that you can `std::ranges::find(rng, boost::none)` for a range of optional objects.
+
+* *Warning.* In the future releases we intend to introduce the range interface
+  in `optional`, so that `std::ranges::range<optional<T>>` will be `true`.
+  This may affect the overload resolution in programs that make decisions based
+  on predicates such as `std::ranges::range`. For instance, the following code
+  will start behaving differently:
+
+  ```
+  template <typename T>
+  void serialize(T const& v)
+  {
+    if constexpr (std::ranges::range<T>)
+      serialize_as_range(v);
+    else if constexpr (custom::is_optional_like<T>)
+      serialize_as_optional(v);
+    else
+      serialize_as_value(v);
+  }
+  ```
 
 [heading Boost Release 1.87]
 

include/boost/none_t.hpp

@@ -25,7 +25,7 @@
 
 #elif defined(BOOST_NO_CXX11_REF_QUALIFIERS) || defined(BOOST_NO_CXX11_NOEXCEPT) || defined(BOOST_NO_CXX11_DEFAULTED_MOVES)
 
-BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.Optional 1.83 and will be removed in Boost.Optional 1.88.")
+BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.Optional 1.83 and will be removed in Boost.Optional 1.92.")
 
 #endif
 
@@ -46,6 +46,11 @@ struct none_t
 {
   struct init_tag{};
   explicit BOOST_CONSTEXPR none_t(init_tag){} // to disable default constructor
+
+#ifndef BOOST_OPTIONAL_DISABLE_EQUALITY_FOR_NONE
+  friend BOOST_CONSTEXPR bool operator==(none_t, none_t) { return true; }
+  friend BOOST_CONSTEXPR bool operator!=(none_t, none_t) { return false; }
+#endif
 };
 
 #endif // old implementation workarounds

include/boost/optional/detail/optional_common_defs.hpp

@@ -22,6 +22,7 @@
 #include <boost/config.hpp>
 #include <boost/core/addressof.hpp>
 #include <type_traits>
+#include <boost/optional/detail/optional_factory_support.hpp>
 
 #ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 #include <boost/type_traits/decay.hpp>

include/boost/optional/detail/union_optional.hpp

@@ -74,9 +74,20 @@ union constexpr_union_storage_t
 
     constexpr constexpr_union_storage_t( trivial_init_t ) noexcept : dummy_() {};
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+// false positive, see https://github.com/boostorg/variant2/issues/55,
+//   https://github.com/boostorg/url/issues/979
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
     template <class... Args>
     constexpr constexpr_union_storage_t( Args&&... args ) : value_(forward_<Args>(args)...) {}
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic pop
+#endif
+
     //~constexpr_union_storage_t() = default; // No need to destroy a trivially-destructible type
 };
 
@@ -88,9 +99,20 @@ union fallback_union_storage_t
 
   constexpr fallback_union_storage_t( trivial_init_t ) noexcept : dummy_() {};
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+// false positive, see https://github.com/boostorg/variant2/issues/55,
+//   https://github.com/boostorg/url/issues/979
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
   template <class... Args>
   constexpr fallback_union_storage_t( Args&&... args ) : value_(forward_<Args>(args)...) {}
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 7)
+# pragma GCC diagnostic pop
+#endif
+
   ~fallback_union_storage_t(){} // My owner will destroy the `T` if needed.
                                 // Cannot default in a union with nontrivial `T`.
 };
@@ -122,6 +144,15 @@ struct constexpr_guarded_storage
     BOOST_CXX14_CONSTEXPR void reset () noexcept { init_ = false; }
 
     //~constexpr_guarded_storage() = default;
+
+#if (defined(_MSC_VER) && 1910 <= _MSC_VER && _MSC_VER <= 1916)
+  // Workaround for MSVC 14.1x bug where it eagerly tries to define the copy/move operations
+  // these are declared but never defined
+    constexpr_guarded_storage(const constexpr_guarded_storage&);
+    constexpr_guarded_storage(constexpr_guarded_storage&&);
+    constexpr_guarded_storage& operator=(const constexpr_guarded_storage&);
+    constexpr_guarded_storage& operator=(constexpr_guarded_storage&&);
+#endif
 };
 
 
@@ -155,6 +186,15 @@ struct fallback_guarded_storage
     }
 
     ~fallback_guarded_storage() { if (init_) storage_.value_.T::~T(); }
+
+#if (defined(_MSC_VER) && 1910 <= _MSC_VER && _MSC_VER <= 1916)
+// Workaround for MSVC 14.1x bug where it eagerly tries to define the copy/move operations
+// These are declared but never defined
+  fallback_guarded_storage(const fallback_guarded_storage&);
+  fallback_guarded_storage(fallback_guarded_storage&&);
+  fallback_guarded_storage& operator=(const fallback_guarded_storage&);
+  fallback_guarded_storage& operator=(fallback_guarded_storage&&);
+#endif
 };
 
 
@@ -195,6 +235,31 @@ namespace boost {
       storage.init_ = true;
     }
 
+  #ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
+    // The conditional initialization of storage needs to employ a factory
+    // function, so that we can utilize the guaranteed copy elision on the
+    // return type.
+    // an alternative -- using a conditional operator in the initializer --
+    // does not work in MSVC 14.2 and 14.3.
+    template <typename... U>
+    static constexpr storage_t conditional_storage_from_values(bool cond, U&&... v)
+    {
+      if (cond)
+        return storage_t(in_place_init, optional_detail::forward_<U>(v)...);
+      else
+        return storage_t();
+    }
+
+    template <typename OU>
+    static constexpr storage_t conditional_storage_from_optional(OU&& ou)
+    {
+      if (ou.has_value())
+        return storage_t(in_place_init, *optional_detail::forward_<OU>(ou));
+      else
+        return storage_t();
+    }
+  #endif
+
   public:
     using value_type = T;
     using unqualified_value_type = typename ::std::remove_const<T>::type;
@@ -218,35 +283,35 @@ namespace boost {
 
 #ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
     constexpr optional(bool cond, const T& v)
-    : storage(cond ? storage_t(v) : storage_t())
+    : storage(conditional_storage_from_values(cond, v))
     {}
 
     constexpr optional(bool cond, T&& v)
-    : storage(cond ? storage_t(optional_detail::move_(v)) : storage_t())
+    : storage(conditional_storage_from_values(cond, optional_detail::move_(v)))
     {}
 
     constexpr optional(const optional& rhs)
-    : storage(rhs.is_initialized() ? storage_t(*rhs) : storage_t())
+    : storage(conditional_storage_from_optional(rhs))
     {}
 
     constexpr optional(optional&& rhs)
       noexcept(::std::is_nothrow_move_constructible<T>::value)
-    : storage(rhs.is_initialized() ? storage_t(*optional_detail::move_(rhs)) : storage_t())
+    : storage(conditional_storage_from_optional(optional_detail::move_(rhs)))
     {}
 
     template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U const&>)>
     constexpr explicit optional(optional<U> const& rhs)
-    : storage(rhs.is_initialized() ? storage_t(in_place_init, *rhs) : storage_t())
+    : storage(conditional_storage_from_optional(rhs))
     {}
 
     template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U&&>)>
     constexpr explicit optional(optional<U> && rhs)
-    : storage(rhs.is_initialized() ? storage_t(in_place_init, *optional_detail::move_(rhs)) : storage_t())
+    : storage(conditional_storage_from_optional(optional_detail::move_(rhs)))
     {}
 
     template <typename... Args>
     constexpr explicit optional( in_place_init_if_t, bool cond, Args&&... args )
-    : storage( cond ? storage_t(in_place_init, optional_detail::forward_<Args>(args)...) : storage_t() )
+    : storage(conditional_storage_from_values(cond, optional_detail::forward_<Args>(args)...))
     {}
 #else
     optional(bool cond, const T& v)
@@ -319,12 +384,14 @@ namespace boost {
     explicit optional (FT&& factory)
     : storage()
     {
-      factory.template apply<T>(this->dataptr());
+      boost_optional_detail::construct<value_type>(factory, this->dataptr());
       storage.init_ = true;
     }
 
     template <typename U,
-              BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U&&>)>
+              BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U&&>),
+              BOOST_OPTIONAL_REQUIRES(!optional_detail::is_typed_in_place_factory<U>),
+              BOOST_OPTIONAL_REQUIRES(!optional_detail::is_in_place_factory<U>)>
     constexpr explicit optional(U&& v)
     : storage(optional_ns::in_place_init, optional_detail::forward_<U>(v))
     {}
@@ -334,6 +401,20 @@ namespace boost {
     : storage(in_place_init, optional_detail::forward_<Args>(args)...)
     {}
 
+    BOOST_CXX14_CONSTEXPR operator optional<T&>() & noexcept
+    {
+      return this->has_value() ? optional<T&>(**this) : optional<T&>();
+    }
+
+    BOOST_CONSTEXPR operator optional<const T&>() const& noexcept
+    {
+      return this->has_value() ? optional<const T&>(**this) : optional<const T&>();
+    }
+
+    BOOST_CXX14_CONSTEXPR operator optional<T&>() && noexcept = delete;
+    BOOST_CONSTEXPR operator optional<const T&>() const&& noexcept = delete;
+
+
     BOOST_CXX14_CONSTEXPR void reset() noexcept
     {
       storage.reset();
@@ -438,7 +519,9 @@ namespace boost {
               BOOST_OPTIONAL_REQUIRES(!::std::is_same<typename ::std::decay<U>::type, optional>),
               BOOST_OPTIONAL_REQUIRES(!optional_detail::conjunction<::std::is_scalar<T>, ::std::is_same<T, BOOST_OPTIONAL_DECAY(U)>>),
               BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U>),
-              BOOST_OPTIONAL_REQUIRES(::std::is_assignable<T&, U>)
+              BOOST_OPTIONAL_REQUIRES(::std::is_assignable<T&, U>),
+              BOOST_OPTIONAL_REQUIRES(!optional_detail::is_typed_in_place_factory<U>),
+              BOOST_OPTIONAL_REQUIRES(!optional_detail::is_in_place_factory<U>)
              >
     BOOST_OPTIONAL_CXX20_CONSTEXPR optional& operator=(U&& v)
     {
@@ -454,7 +537,7 @@ namespace boost {
     optional& operator=(F&& factory)
     {
       reset();
-      factory.template apply<T>(this->dataptr());
+      boost_optional_detail::construct<value_type>(factory, this->dataptr());
       storage.init_ = true;
       return *this;
     }

include/boost/optional/optional.hpp

@@ -84,10 +84,8 @@ operator<<(std::basic_ostream<CharType, CharTrait>& os, optional_detail::optiona
 #include <boost/type_traits/is_volatile.hpp>
 #include <boost/type_traits/is_scalar.hpp>
 
-
 #include <boost/optional/optional_fwd.hpp>
 #include <boost/optional/detail/optional_config.hpp>
-#include <boost/optional/detail/optional_factory_support.hpp>
 #include <boost/optional/detail/optional_aligned_storage.hpp>
 
 

test/Jamfile.v2

@@ -28,7 +28,6 @@ project
             cxx11_explicit_conversion_operators
 #            cxx11_noexcept
             cxx11_rvalue_references
-            cxx11_static_assert
             cxx11_variadic_templates
         ]
     ;
@@ -49,6 +48,7 @@ run optional_test_flat_map.cpp ;
 run optional_test_hash.cpp ;
 run optional_test_map.cpp ;
 run optional_test_tie.cpp : : : <library>/boost/tuple//boost_tuple ;
+run optional_test_ranges_find.cpp ;
 run optional_test_ref_assign_portable_minimum.cpp ;
 run optional_test_ref_assign_mutable_int.cpp ;
 run optional_test_ref_assign_const_int.cpp ;
@@ -80,6 +80,7 @@ compile-fail optional_test_fail3b.cpp ;
 compile-fail optional_test_ref_fail1.cpp ;
 compile-fail optional_test_ref_fail3.cpp ;
 compile-fail optional_test_ref_fail4.cpp ;
+compile-fail optional_test_ref_fail_convert_from_temporary_optional_T.cpp ;
 compile-fail optional_test_inplace_fail.cpp ;
 compile-fail optional_test_inplace_fail2.cpp ;
 compile-fail optional_test_fail_implicit_bool_convert.cpp ;

test/optional_test_constexpr.cpp

@@ -20,6 +20,16 @@ struct Record
   constexpr int operator()() const { return i; }
 };
 
+struct Guard
+{
+  int i;
+  constexpr explicit Guard(int i) : i(i) {}
+  Guard(Guard&&) = delete;
+};
+
+static_assert(boost::none == boost::none, "");
+static_assert(!(boost::none != boost::none), "");
+
 namespace test_int
 {
   constexpr boost::optional<int> oN;
@@ -52,7 +62,7 @@ namespace test_int
   static_assert(o2 != o1, "");
   static_assert(o2 > o1, "");
 
-#ifndef BOOST_NO_CXX14_CONSTEXPR
+#ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
   constexpr boost::optional<int> oNc = oN;
   constexpr boost::optional<int> oNd = boost::none;
   constexpr boost::optional<int> oNe = {};
@@ -88,6 +98,29 @@ namespace test_record
   static_assert(r2.value().i == 2, "");
 }
 
+namespace test_guard
+{
+  constexpr boost::optional<Guard> g1 {boost::in_place_init, 1};
+  constexpr boost::optional<Guard> gNa {boost::none};
+  constexpr boost::optional<Guard> gNb;
+
+  static_assert(g1, "");
+  static_assert(!!g1, "");
+  static_assert(g1 != boost::none, "");
+  static_assert(!(g1 == boost::none), "");
+  static_assert(g1.has_value(), "");
+
+  static_assert(!gNa, "");
+  static_assert(!gNa.has_value(), "");
+  static_assert(gNa == boost::none, "");
+  static_assert(!(gNa != boost::none), "");
+
+  static_assert(!gNb, "");
+  static_assert(!gNb.has_value(), "");
+  static_assert(gNb == boost::none, "");
+  static_assert(!(gNb != boost::none), "");
+}
+
 namespace test_optional_ref
 {
   constexpr int gi = 9;

test/optional_test_convert_from_T.cpp

@@ -45,6 +45,8 @@ void test_optional_of_superconverting_T() // compile-time test
   superconv<optional<int> > s;
    superconv<optional<int> > & rs = s;
   optional<superconv<optional<int> > > os = rs;
+  (void)s;
+  (void)os;
 #endif
 }
 

test/optional_test_inplace_factory.cpp

@@ -105,9 +105,42 @@ void test_assign()
 #endif
 }
 
+// begin Boost.Log case
+
+template <typename CharT>
+struct basic_formatter
+{
+  template< typename FunT >
+  basic_formatter(FunT&&) {}
+
+  template< typename FunT >
+  basic_formatter& operator= (FunT&&)
+  {
+      return *this;
+  }
+};
+
+template< typename CharT>
+struct chained_formatter
+{
+};
+
+void test_boost_log_case()
+{
+#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
+
+  boost::optional<basic_formatter<char>> of( boost::in_place(chained_formatter<char>()) );
+  of = boost::in_place(chained_formatter<char>());
+
+#endif //BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
+}
+
+// end Boost.Log case
+
 int main()
 {
   test_ctor();
   test_assign();
+  test_boost_log_case();
   return boost::report_errors();
 }

test/optional_test_msvc_bug_workaround.cpp

@@ -32,6 +32,7 @@ void test()
   //BOOST_TEST(v);
   boost::optional<boost::optional<int>> vv;
   bool xx = vv?true : false;
+  (void)xx;
   BOOST_TEST_EQ(*v, 7);
 #endif
 }

test/optional_test_ranges_find.cpp

@@ -0,0 +1,70 @@
+// Copyright (C) 2026 Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+
+
+#include "boost/optional/optional.hpp"
+#include "boost/core/lightweight_test.hpp"
+#include <type_traits>
+
+#ifndef BOOST_NO_CXX20_HDR_RANGES
+#include <ranges>
+#include <concepts>
+#include <iterator>
+#include <array>
+#include <string>
+
+static_assert(std::equality_comparable<boost::none_t>, "boost::none shall be equality comparable");
+
+
+template <typename T>
+void test_that_you_can_find_none_in_a_range_of_optional(T x, T y)
+{
+  static_assert(std::equality_comparable_with<boost::optional<T>, boost::none_t>, "boost::none shall satisfy the concept");
+  static_assert(std::equality_comparable_with<boost::none_t, boost::optional<T> >, "boost::none shall satisfy the concept");
+  static_assert(std::indirect_binary_predicate<std::ranges::equal_to, const boost::optional<T>*, const boost::none_t*>, "boost::none shall satisfy the concept");
+
+  //                                        [0]     [1]     [2]
+  std::array<boost::optional<T>, 3> arr = {{ x, boost::none, y }};
+  auto it = std::ranges::find(arr, boost::none);
+  BOOST_TEST_EQ(std::distance(arr.begin(), it), 1);
+}
+#endif // BOOST_NO_CXX20_HDR_RANGES
+
+
+# if (defined __GNUC__) && (!defined BOOST_INTEL_CXX_VERSION) && (!defined __clang__)
+#  if (__GNUC__ <= 4)
+#   define BOOST_OPTIONAL_TEST_INSUFFICIENT_TYPE_TRAIT_SUPPORT
+#  endif
+# endif
+
+#ifndef BOOST_OPTIONAL_TEST_INSUFFICIENT_TYPE_TRAIT_SUPPORT
+static_assert(std::is_trivially_copyable<boost::none_t>::value, "boost::none shall be trivially copyable");
+#endif
+
+
+void test_that_none_is_equal_to_none()
+{
+  BOOST_TEST(boost::none == boost::none);
+  BOOST_TEST(!(boost::none != boost::none));
+  auto None = boost::none;
+  BOOST_TEST(None == boost::none);
+  BOOST_TEST(!(None != boost::none));
+}
+
+int main()
+{
+#ifndef BOOST_NO_CXX20_HDR_RANGES
+  test_that_you_can_find_none_in_a_range_of_optional(1, 2);
+  test_that_you_can_find_none_in_a_range_of_optional<std::string>("one", "two");
+#endif
+  test_that_none_is_equal_to_none();
+  return boost::report_errors();
+}

test/optional_test_ref_fail_convert_from_temporary_optional_T.cpp

@@ -0,0 +1,21 @@
+// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  fernando_cacciola@hotmail.com
+//
+#include "boost/optional.hpp"
+
+//
+// THIS TEST SHOULD FAIL TO COMPILE
+//
+void optional_reference__test_no_converting_initialization()
+{
+  boost::optional<const int&> o (boost::optional<int>(1));
+  (void)o;
+}

test/optional_test_ref_to_val.cpp

@@ -24,7 +24,13 @@ using boost::none;
 struct Value
 {
   int val;
-  explicit Value(int v) : val(v) {}
+  BOOST_CONSTEXPR explicit Value(int v) : val(v) {}
+};
+
+struct Guard
+{
+  int val;
+  BOOST_CONSTEXPR explicit Guard(int v) : val(v) {}
 };
 
 int val(int const& i)
@@ -37,6 +43,11 @@ int val(Value const& v)
   return v.val;
 }
 
+int val(Guard const& v)
+{
+  return v.val;
+}
+
 template <typename Tref>
 optional<Tref&> make_opt_ref(Tref& v)
 {
@@ -47,21 +58,21 @@ template <typename Tval, typename Tref>
 void test_construct_from_optional_ref()
 {
     Tref v1 (1), v2 (2);
-    
+
     optional<Tref&> opt_ref0;
     optional<Tref&> opt_ref1 (v1);
-    
+
     optional<Tval> opt_val0 (opt_ref0);
     optional<Tval> opt_val1 (opt_ref1);
     optional<Tval> opt_val2 (make_opt_ref(v2));
-    
+
     BOOST_TEST (!opt_val0);
     BOOST_TEST (opt_val1);
     BOOST_TEST (opt_val2);
-    
+
     BOOST_TEST_EQ (1, val(*opt_val1));
     BOOST_TEST_EQ (2, val(*opt_val2));
-    
+
     BOOST_TEST (boost::addressof(*opt_val1) != boost::addressof(v1));
     BOOST_TEST (boost::addressof(*opt_val2) != boost::addressof(v2));
 }
@@ -70,29 +81,121 @@ template <typename Tval, typename Tref>
 void test_assign_from_optional_ref()
 {
     Tref v1 (1), v2 (2);
-    
+
     optional<Tref&> opt_ref0;
     optional<Tref&> opt_ref1 (v1);
-    
+
     optional<Tval> opt_val0;
     optional<Tval> opt_val1;
     optional<Tval> opt_val2;
-    
+
     opt_val0 = opt_ref0;
     opt_val1 = opt_ref1;
     opt_val2 = make_opt_ref(v2);
-    
+
     BOOST_TEST (!opt_val0);
     BOOST_TEST (opt_val1);
     BOOST_TEST (opt_val2);
-    
+
     BOOST_TEST_EQ (1, val(*opt_val1));
     BOOST_TEST_EQ (2, val(*opt_val2));
-    
+
     BOOST_TEST (boost::addressof(*opt_val1) != boost::addressof(v1));
     BOOST_TEST (boost::addressof(*opt_val2) != boost::addressof(v2));
 }
 
+template <typename T>
+void test_convert_optional_T_to_optional_T_ref()
+{
+#ifdef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+  using boost::optional;
+  using boost::in_place_init;
+
+  { // optional<T>& -> optional<T&>
+    optional<T> ovN, ov1(in_place_init, 1), ov2(in_place_init, 2);
+
+    optional<T&> orN = ovN;
+    optional<T&> or1 = ov1;
+    optional<T&> or2 = ov2;
+
+    BOOST_TEST_EQ (!!orN, !!ovN);
+    BOOST_TEST_EQ (!!or1, !!ov1);
+    BOOST_TEST_EQ (!!or2, !!ov2);
+
+    BOOST_TEST (or1);
+    BOOST_TEST (or2);
+    BOOST_TEST_EQ (val(*or1), 1);
+    BOOST_TEST_EQ (val(*or2), 2);
+    BOOST_TEST (boost::addressof(*or1) == boost::addressof(*ov1));
+    BOOST_TEST (boost::addressof(*or2) == boost::addressof(*ov2));
+  }
+
+  { // const optional<T>& -> optional<const T&>
+    constexpr optional<T> ovN;
+    constexpr optional<T> ov1(in_place_init, 1);
+    constexpr optional<T> ov2(in_place_init, 2);
+
+    optional<const T&> orN = ovN;
+    optional<const T&> or1 = ov1;
+    optional<const T&> or2 = ov2;
+
+    BOOST_TEST_EQ (!!orN, !!ovN);
+    BOOST_TEST_EQ (!!or1, !!ov1);
+    BOOST_TEST_EQ (!!or2, !!ov2);
+
+    BOOST_TEST (or1);
+    BOOST_TEST (or2);
+    BOOST_TEST_EQ (val(*or1), 1);
+    BOOST_TEST_EQ (val(*or2), 2);
+    BOOST_TEST (boost::addressof(*or1) == boost::addressof(*ov1));
+    BOOST_TEST (boost::addressof(*or2) == boost::addressof(*ov2));
+  }
+
+  { // optional<const T>& -> optional<const T&>
+    optional<const T> ovN;
+    optional<const T> ov1(in_place_init, 1);
+    optional<const T> ov2(in_place_init, 2);
+
+    optional<const T&> orN = ovN;
+    optional<const T&> or1 = ov1;
+    optional<const T&> or2 = ov2;
+
+    BOOST_TEST_EQ (!!orN, !!ovN);
+    BOOST_TEST_EQ (!!or1, !!ov1);
+    BOOST_TEST_EQ (!!or2, !!ov2);
+
+    BOOST_TEST (or1);
+    BOOST_TEST (or2);
+    BOOST_TEST_EQ (val(*or1), 1);
+    BOOST_TEST_EQ (val(*or2), 2);
+    BOOST_TEST (boost::addressof(*or1) == boost::addressof(*ov1));
+    BOOST_TEST (boost::addressof(*or2) == boost::addressof(*ov2));
+  }
+
+  { // optional<T>& -> optional<const T&>
+    optional<T> ovN;
+    optional<T> ov1(in_place_init, 1);
+    optional<T> ov2(in_place_init, 2);
+
+    optional<const T&> orN = ovN;
+    optional<const T&> or1 = ov1;
+    optional<const T&> or2 = ov2;
+
+    BOOST_TEST_EQ (!!orN, !!ovN);
+    BOOST_TEST_EQ (!!or1, !!ov1);
+    BOOST_TEST_EQ (!!or2, !!ov2);
+
+    BOOST_TEST (or1);
+    BOOST_TEST (or2);
+    BOOST_TEST_EQ (val(*or1), 1);
+    BOOST_TEST_EQ (val(*or2), 2);
+    BOOST_TEST (boost::addressof(*or1) == boost::addressof(*ov1));
+    BOOST_TEST (boost::addressof(*or2) == boost::addressof(*ov2));
+  }
+
+#endif // BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+}
+
 
 int main()
 {
@@ -100,17 +203,21 @@ int main()
     test_construct_from_optional_ref<int, int const>();
     test_construct_from_optional_ref<int const, int const>();
     test_construct_from_optional_ref<int const, int>();
-    
+
     test_construct_from_optional_ref<Value, Value>();
     test_construct_from_optional_ref<Value, Value const>();
     test_construct_from_optional_ref<Value const, Value const>();
     test_construct_from_optional_ref<Value const, Value>();
-    
+
     test_assign_from_optional_ref<int, int>();
     test_assign_from_optional_ref<int, int const>();
 
     test_assign_from_optional_ref<Value, Value>();
     test_assign_from_optional_ref<Value, Value const>();
 
+    test_convert_optional_T_to_optional_T_ref<int>();
+    test_convert_optional_T_to_optional_T_ref<Value>();
+    test_convert_optional_T_to_optional_T_ref<Guard>();
+
     return boost::report_errors();
 }